I've been really impressed with these, and they're big enough to cover most holes from the old light.
http://www.superbrightleds.com/moreinfo/dome-lights/led-round-dome-light-led-fixture-3w/1544/3514/
http://www.superbrightleds.com/moreinfo/dome-lights/led-round-dome-light-led-fixture-3w/1544/3514/